Heading Sensor for Deflector Angle of Attack Estimation

ABSTRACT

Embodiments relate to coupling a heading sensor to a deflector surface reference for obtaining the deflector heading which can be used in estimation of the deflector angle of attack. A method may comprise: towing a plurality of streamers behind a survey vessel in a body of water, wherein at least one deflector provides a lateral component of force to the streamers; determining a deflector heading over ground using at least measurements from a heading sensor on a surface reference corresponding to the deflector; determining a deflector velocity over ground using at least measurements from a position sensor on the surface reference; determining a water current of the body of water; and estimating a deflector angle of attack based on inputs comprising the deflector heading, the deflector velocity over ground, and the water current.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 62/013,786, filed on Jun. 18, 2014, the entire disclosure of whichis incorporated herein by reference.

BACKGROUND

Embodiments relate generally to marine geophysical surveying and, moreparticularly, embodiments relate to coupling a heading sensor to adeflector surface reference for obtaining the deflector heading whichcan be used in estimation of the deflector angle of attack.

Techniques for geophysical surveying include marine geophysicalsurveying, such as seismic surveying and electromagnetic surveying, inwhich geophysical data may be collected from below the Earth's surface.Geophysical surveying has applications in mineral and energy explorationand production to help identify locations of hydrocarbon-bearingformations. Certain types of marine geophysical surveying, such asseismic or electromagnetic surveying, may include towing an energysource at a selected depth—typically above the seafloor—in a body ofwater. One or more streamers also may be towed in the water at selecteddepths—typically above the seafloor—by the same or a different vessel.The streamers are typically cables that include a plurality ofgeophysical sensors disposed thereon at spaced apart locations along thelength of the cable. Some geophysical surveys locate sensors on oceanbottom cables or nodes in addition to, or instead of, streamers. Thegeophysical sensors may be configured to generate a signal that isrelated to a parameter being measured by the geophysical sensor. Atselected times, the energy source may be actuated to generate, forexample, seismic or electromagnetic energy that travels downwardly intothe subsurface rock. Energy that interacts with interfaces, generally atthe boundaries between layers of rock formations, may be returned towardthe surface and detected by the geophysical sensors on the streamers.The detected energy may be used to infer certain properties of thesubsurface rock, such as structure, mineral composition and fluidcontent, thereby providing information useful in the recovery ofhydrocarbons.

Current marine geophysical survey techniques may utilize multiplestreamers towed at selected lateral distances from one another.Spreading devices are commonly used in geophysical surveying to achievethe desired lateral spread between the streamers. The spreading devicesmay include a variety of devices, such as doors, paravanes, steeringrudders, collectively referred to herein as “deflectors.” Vessel motionsand water currents can produce a rather high variance of uncertaintiesin the estimation of deflector angle of attack.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present invention and should not be used to limit or define theinvention.

FIG. 1 illustrates an example embodiment of a geophysical survey system.

FIG. 2 illustrates an example embodiment of a deflector with acorresponding surface reference that includes a heading sensor and aglobal positioning system.

FIG. 3 illustrates an example embodiment of deflector showing its angleof attack.

FIG. 4 illustrates an example embodiment of a data processing systemthat may be used in a deflector angle of attack estimation.

FIG. 5 illustrates an example data processing system and its respectiveinputs and outputs in a deflector angle of attack estimation.

DETAILED DESCRIPTION

It is to be understood the present disclosure is not limited toparticular devices or methods, which may, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular embodiments only and is not intended to belimiting. As used herein, the singular forms “a”, “an”, and “the”include singular and plural referents unless the content clearlydictates otherwise. Furthermore, the word “may” is used throughout thisapplication in a permissive sense (i.e., having the potential to, beingable to), not in a mandatory sense (i.e., must). The term “include”, andderivations thereof, mean “including, but not limited.” The term“coupled” means directly or indirectly connected.

Embodiments relate generally to marine geophysical surveying. Forexample, the embodiments disclosed herein may have applications inmarine seismic surveying, in which one or more seismic sources may beused to generate seismic energy that interacts with subsurfaceformations, and sensors—either towed or ocean bottom—may receive seismicenergy generated by the seismic sources and affected by interaction withthe subsurface formations. Likewise, the embodiments disclosed hereinmay also have applications in marine electromagnetic surveying, in whichone or more electromagnetic sources may be used to generateelectromagnetic fields that interact with subsurface formations, andelectromagnetic sensors—either towed or ocean bottom—may receive theelectromagnetic energy affected by the interaction with the subsurfaceformations.

Examples of the present embodiments may include use of a heading sensorsuch as a compass, gyroscope, or magnetometer for an angle of attackestimation of a deflector. For example, in a number of embodiments, aheading sensor may be coupled to a surface reference corresponding to adeflector with its heading measurements combined with water currentmeasurements and position measurements in order to estimate an angle ofattack of a deflector. Advantageously, by incorporation of the headingsensor into the surface reference—rather than the deflectoritself—certain benefits may be achieved. For example, magneticdisturbances in the heading measurements may be minimized by placementon the surface reference, as motors or other equipment on the deflectorcan interfere with the heading sensor. By way of further example, spaceconstraints on the deflector may make placement of the heading sensor onthe deflector problematic, while the surface reference could includecompartments or other areas for positioning of the heading sensor.Similar benefits may also be obtained by incorporation of the positionsensor for position measurements into the surface reference rather thanthe deflector. In specific embodiments, the water current measurementsmay be taken at the survey vessel rather than at the deflector. By usingthe existing water current measurement system on the survey vessel, theadditional expense of complexity of incorporating this system at thedeflector may be avoided while also allowing for ease of service andreplacement.

An example method may comprise towing a plurality of streamers behind asurvey vessel in a body of water, wherein at least one deflectorprovides a lateral component of force to the streamers; determining adeflector heading over ground using at least measurements from a headingsensor on a surface reference corresponding to the deflector;determining a deflector velocity over ground using at least measurementsfrom a position sensor on the surface reference; determining a watercurrent of the body of water; and estimating a deflector angle of attackbased on inputs comprising the deflector heading, the deflector velocityover ground, and the water current.

An example system may comprise: a heading sensor on a surface referencecorresponding to a deflector; a position sensor on the surfacereference; a current measurement system on a survey vessel; and a dataprocessing system communicatively coupled to the heading sensor,position sensor, and current measurement system, wherein the dataprocessing system is configured to estimate a deflector angle of attackfor the deflector based on inputs comprising measurements from theheading sensor indicative of deflector heading, measurements from theposition sensor indicative of deflector velocity over ground, andmeasurements from the current measurement system indicative of watercurrent.

An example system may comprise: a survey vessel; a current measurementsystem on the survey vessel; a streamer coupled to the survey vessel fortowing behind the survey vessel, wherein geophysical sensors aredisposed on the streamer at spaced apart locations; a deflector coupledto the streamer; a surface reference coupled to the deflector forsupporting the deflector in a body of water; a heading sensor on thesurface reference; and a position sensor on the surface reference.

Referring now to FIG. 1, a marine geophysical survey system 100 isillustrated in accordance with example embodiments. As illustrated, themarine geophysical survey system 100 may include a survey vessel 105moving along the surface of a body of water 110, such as a lake orocean. The survey vessel 105 may include thereon equipment, showngenerally at 115 and referred to for convenience as a “recordingsystem.” The recording system 115 typically may include devices (noneshown separately) for navigating the survey vessel 105 (such as globalpositioning system (“GPS”) receivers), for actuating at least one energysource 120, and/or for recording signals generated by geophysicalsensors 125. The survey vessel may further include a water currentmeasurement system 118, which may be used for determining the localwater current of the body of water 110. A suitable water currentmeasurement system 118 may include a variety of different devices,including a global positioning system, an acoustic pinger, and/or amagnetometer/gyroscope, which may provide information, such as vesselheading and vessel orientation, for determination of the water velocity.An example of a suitable water current measurement system 118 includesan acoustic Doppler current profiler (sometimes referred to as an ADCPor ADP), which uses acoustic beams to measure the water velocityremotely from the survey vessel 105.

As illustrated, the survey vessel 105 (or a different vessel) may towthe energy source 120 in the body of water 110. During operation, theenergy source 120 may be triggered at selected times. When triggered,the energy source 120 may produce energy that emanates outwardly fromthe energy source 120. The energy may travel downwardly through the bodyof water 110 and into rock formations below the water bottom. A sourcecable 130 may couple the energy source 120 to the survey vessel 105. Inthe illustrated embodiment, the energy source 120 is towed below thesurface of the body of water 110. As illustrated, the energy source 120may be below the surface of the body of water 110 and above the waterbottom, wherein the energy source 120 may be disconnected from the waterbottom. For example, the energy source 120 may be towed in the body ofwater 110 at a depth ranging from 0 meters to about 300 meters. Theenergy source 120 may be any selectively actuable source suitable formarine geophysical surveying, including without limitation a seismic airgun, a water gun, a marine vibrator, an electromagnetic fieldtransmitter, or an array of such devices. In some embodiments, seismicenergy and/or electromagnetic energy may originate from the energysource 120. The energy source 120 may be towed in any suitable patternfor geophysical surveying, including in a parallel or orthogonalpattern, or possibly a circular or spiral pattern. It should be notedthat, while the present example shows only a single energy source 120,the invention is applicable to any number of energy sources towed by thesurvey vessel 105 or any other vessel.

As illustrated in FIG. 1, the marine geophysical survey system 100 mayfurther include streamers 135, which may be spaced apart laterallyand/or vertically. “Lateral” or “laterally,” in the present context,means transverse to the direction of the motion of the survey vessel105. Geophysical sensors 125 may be disposed on the streamers 135 atspaced apart locations. The streamers 135 may each be formed, forexample, by coupling a plurality of streamer segments end-to-end asexplained in U.S. Pat. No. 7,142,481, the disclosure of which isincorporated herein by reference. In one embodiment, streamers 135 mayeach include one or more lateral force and depth (“LFD”) control devices(not shown). The LFD control devices may be deployed, for example, toregulate streamer depth so that the streamers 135 may be kept at aselected depth profile (e.g., as level as possible) while towed throughthe body of water 110. The LFD control device may be any of a variety ofdifferent devices suitable for regulating streamer depth, including“birds” having variable-incidence wings. It should be noted that, whilethe present example, shows only six streamers 135, the invention isapplicable to any number of laterally spaced apart streamers 135 towedby survey vessel 105 or any other vessel. For example, in someembodiments, 8 or more laterally spaced apart streamers 135 may be towedby survey vessel 105, while in other embodiments, up to 26 laterallyspaced apart streamers 135 may be towed by survey vessel 105.

In the illustrated embodiment, the streamers 135 may be coupled to thesurvey vessel 105 using a one or more lead-in lines, such as lead-inlines 140. The lead-in lines 140 may be used, for example, to deploy thestreamers 135 from the survey vessel 105 and to maintain the streamers135 at a selected distance behind the survey vessel 105. As illustrated,the lead-in lines 140 may be coupled at one end to the survey vessel 105and at the other end to the corresponding one of the streamers 135. Insome embodiments, a lead-in line 140 may couple to multiple streamers135. The lead-in lines 140 may be, for example, any of a variety ofspoolable lines suitable for use in geophysical survey systems,including, without limitation, fiber ropes, armored cables, or anysimilar device or combination thereof.

In the illustrated embodiment, the streamers 135 may be coupled at theirforward ends to one or more spreader lines 145, which extend between thestreamers 135. As illustrated, the spreader lines 145 may interconnectthe streamers 135. In general, the spreader lines 145 may extend in thebody of water 110 essentially transversely to the direction of motion ofthe survey vessel 105. For example, when maintained in correct tension,spreader lines 145 may help to maintain the lateral positions of theforward ends of the streamers 135. The spreader lines 145 may be, forexample, any of a variety of lines suitable for use in electromagneticsurvey systems, including, without limitation, fiber ropes, armoredcables, or any similar device or combination thereof. While FIG. 1illustrates a particular towing configuration using lead-in lines 140and spreader lines 145, it should be understood that other towingconfigurations that may use more or less lines and/or differentarrangements thereof may be used in accordance with present embodiments.

The geophysical sensors 125 may be disposed at spaced apart locations onthe streamers 135. The geophysical sensors 125 may be any type of sensorknown in the art. While not shown, some geophysical surveys may locatethe geophysical sensors 125 on ocean bottom cables or nodes in additionto, or instead of, the streamers 135. The geophysical sensors 125 may beany type of geophysical sensor known in the art, including seismicsensors, such as hydrophones, geophones, particle velocity sensors,particle displacement sensors, particle acceleration sensors, orpressure gradient sensors, or electromagnetic field sensors, such aselectrodes or magnetometers. The geophysical sensors 125 may detectenergy that originated from the energy source 120 after it hasinteracted with the rock formations. By way of example, the geophysicalsensors 125 may generate signals, such as electrical or optical signals,in response to the detected energy. Signals generated by the geophysicalsensors 125 may be communicated to the recording system 115. Thedetected energy may be used to infer certain properties of thesubsurface rock, such as structure, mineral composition and fluidcontent, thereby providing information useful in the recovery ofhydrocarbons.

In accordance with an embodiment of the invention, a geophysical dataproduct may be produced. The geophysical data product may includegeophysical data obtained from one or more of the geophysical sensors125 and may be stored on a non-transitory, tangible computer-readablemedium. The geophysical data product may be produced offshore (i.e. byequipment on a vessel) or onshore (i.e. at a facility on land) eitherwithin the United States or in another country. If the geophysical dataproduct is produced offshore or in another country, it may be importedonshore to a facility in the United States. Once onshore in the UnitedStates, geophysical analysis, including further data processing, may beperformed on the geophysical data product.

As illustrated, the marine geophysical survey system 100 may furtherinclude deflectors 150. The deflectors 150 may be any type of deflectorknown in the art, such as doors, paravanes, steering rudders, and thelike. One example of a suitable deflector includes a wing-shaped bodyused to generate lateral thrust. Another example of a suitable deflectorincludes one or more foils that generate lateral thrust as towed throughthe body of water 110. As illustrated, the deflectors 150 may be coupledto the streamers 135. In one embodiment, spur lines 155 may couple thestreamers 135 to the deflectors 150. The spur lines 155 may be any of avariety of lines suitable for use in electromagnetic survey systems,including, without limitation, fiber ropes, armored cables, or anysimilar device or combination thereof. Deflector lead-in lines 160 maycouple the deflectors 150 to the survey vessel 105. In alternativeembodiments (not shown), deflector lead-in lines 160 are not used. Itshould be noted that, while the present example shows only twodeflectors 150, the invention is applicable to any number of deflectors150 that may be used as desired for a particular application. In someembodiments, the deflectors 150 may be remotely controlled, for example,to control the angle of attack. Deflector angle of attack will bedescribed in more detail below with respect to FIG. 3.

Turning now to FIG. 2, a deflector 150 is shown in more detail inaccordance with an example embodiment. As illustrated, the deflector 150may be towed by a deflector lead-in line 160. The deflector 150 mayinclude a tension sensor 200, such as a strain gauge or load cell,positioned proximate deflector lead-in line 160. The tension sensor 200may be an electrical sensor, such as a Wheatstone bridge type sensor oran optical sensor, such as Bragg grating etched into an optical fiber.The tension sensor 200 may also be a hydrostatic pressure sensor, suchas piezoelectric type sensors. Measurements made by the tension sensor200 may be provided to the recording system 115 (see FIG. 1) foranalysis and control. From the tension sensor 200, deflector lift anddeflector drag may be determined.

As illustrated on FIG. 2, a surface reference 205 may be coupled to thedeflector 150. The surface reference 205 may help to support thedeflector 150 in the body of water 110. As illustrated, the surfacereference 205 may be at the surface of the body of water 110. Any typeof surface reference 205 may be used, including a float, buoy, or othersuitable flotation device.

The surface reference 205 may include a heading sensor 210 and aposition sensor 215. The heading sensor 210 and position sensor 215 maybe coupled to, disposed on, integrated into, or otherwise attached tothe surface reference 205. The heading sensor 210 may include anysuitable sensor for determining heading of the surface reference 205,including without limitation a magnetometer, a gyroscope, or a compass.The heading of the surface reference 205 may generally correspond to theheading of the corresponding deflector 150. The position sensor 215 mayinclude any suitable sensor, such as a global positioning system sensor,which can be used to provide information on the location of the surfacereference 205 as a function of time. The location of the surfacereference 205 may generally correspond with the location of thedeflector 150. From the location information of the surface reference205, the velocity of the deflector 150 over ground can be determined.The heading sensor 210 and position sensor 215 may communicate with therecording system 115 (e.g., FIG. 1) via a wireless link, such as a radiolink. As will be discussed in more detail below, information from theheading sensor 210 may be combined with location information from theposition sensor 215 and water current measurements from the watercurrent measurement system 118 (e.g., FIG. 1) to estimate the angle ofattack of the deflector.

FIG. 3 illustrates the angle of attack 300 for the deflector 150 inaccordance with an example embodiment. As illustrated, the deflector 150may be towed by deflector lead-in line 160. The angle of attack 300,sometimes referred as the “yaw” angle, is the angle formed between thedirection of deflector forward motion 305 and deflector chord line 310.The deflector chord line 310 is an imaginary line that joins the leadingedge 315 and trailing edge 320 of the deflector 150. The deflectorforward motion 305 is the direction of the deflector's movement throughthe body of water 110 in the horizontal plane. The deflector forwardmotion 305 may not necessarily correspond with the direction of thesurvey vessel 105 (e.g., FIG. 1). For example, when the survey vessel105 is turning the deflector forward motion 305 and the direction of thesurvey vessel 105 may differ.

As previously mentioned, measurement of the angle of attack 300 of thedeflector 150 can facilitate operation of the deflector 150 as well ascontrol of the entire survey spread. Knowledge of the angle of attack300 may be beneficial in a number of different aspect for operation ofthe deflector 150, including, without limitation, preventing stall ofdeflector 150 due to too high an angle of attack 300, preventingcollapse of deflector 150 due to low of an angle of attack 300, enablingoperator to use the full range/potential of the deflector 150, andmonitoring of deflector 150 efficiency. The angle of attack 300 may alsobe adjusted when the heading of the survey vessel 105 is changed toincrease or decrease the lateral thrust as desired. Knowledge of theangle of attack may also be beneficial in control of the survey spread.For example, the angle of attack 300 can be adjusted to modify thelateral thrust generated by the deflector 150, thus increasing ordecreasing the spread of the streamers 135 (e.g., FIG. 1) as desired fora particular application.

FIG. 4 illustrates use of example embodiment utilizing a data processingsystem 400 for estimation of the angle of attack. The data processingsystem 400 may include, for example, a processor, memory, andinput/output devices. An example data processing system 400 is describedin more detail in connection with FIG. 5. In some embodiments, the dataprocessing system 400 may be a component of the recording system 115(e.g., FIG. 1). In alternative embodiments, the data processing system400 may be separate from the recording system 115. It should beunderstood that the data processing system 400 does not necessarily haveto be located on the survey vessel 105. The data processing system 400may be in signal communication, which may be wired or wirelesscommunication, with the water current measurement system 118, theheading sensor 210, and the position sensor 215.

The data processing system 400 may be able to determine an estimate ofthe angle of attack 300 of the deflector 150 (e.g., FIG. 3) from anumber of inputs. A first input may include deflector heading 405, whichmay correspond to the heading of the deflector 150 over ground. Thedeflector heading 405 may be determined based at least on measurementsfrom the heading sensor 210 (e.g., FIG. 2). A second input may includedeflector velocity 410, which may correspond to velocity of thedeflector 150 over ground. The deflector velocity 410 may be determinedbased at least on measurements from the position sensor 215 (e.g., FIG.2). A third input may include water current 415, which may correspond tothe water current of the body of water 110 (e.g., FIG. 1). The watercurrent 415 may be determined based at least on measurements from watercurrent measurement system 118 (e.g., FIG. 1). One or more of the inputsmay be provided to the data processing system 400 from another system,for example, a navigation system (not shown) may provide the deflectorvelocity 410 based on the GPS measurements, or one or more of the inputsmay be determined by the data processing system 400 and used indetermining the estimate of the angle of attack of the deflector 150.

Accordingly, the angle of attack 300 may be estimated in accordance withexample embodiments. With the angle of attack 300, operation of thedeflector 150 (e.g., FIG. 3) and of the entire spread of streamers 135(e.g., 1) may be facilitated. In some embodiments, the angle of attack300 and the deflector 150 lift may be compared. With this comparison, itmay be possible to monitor changes in efficiency caused by either afailing component or marine growth on towed equipment, such as thedeflector 150. In response to this comparison, remedial action may betaken. For example, the angle of attack 300 may be increased to returnto the same deflector lift, but may result in increased draft andresulting decreased efficiency. By way of further example, the remedialaction may include cleaning of the deflector 150 such scraping of thedeflector 150 to remove marine growth (e.g., barnacles).

The deflector 150 lift may be determined based on measurements from thetension sensor 200 on the deflector 150 (e.g., FIG. 2). By way ofexample, the lift L can be estimated knowing the tension T from thetension sensor 200 (e.g., FIG. 2) using the known drag/lift relationshipk. The lift can then be calculated according to the following equation:

$\begin{matrix}{L = \sqrt{\frac{T^{2}}{1 + k^{2}}}} & (1)\end{matrix}$

Wherein L is the deflector 150 lift, T is the measured tension, and k isthe known drag/lift relationship.

An example technique for estimating the angle of attack 300 usingdeflector heading 405, deflector velocity 410, and water current 415will now be described in more detail. For simplicity, it is assumed thatthe deflector heading and the direction of the reference line for theangle of attack 300 coincide, and the axis in the ground and the watercoordinate system coincide.

The water current 415 (C) in reference to the ground coordinate systemmay be defined as follows:

$\begin{matrix}{C = \begin{pmatrix}C_{x} \\C_{y}\end{pmatrix}} & (2)\end{matrix}$

The deflector velocity 410 (G) in relation to ground coordinate systemmay be defined as follows:

$\begin{matrix}{G = \begin{pmatrix}G_{x} \\G_{y}\end{pmatrix}} & (3)\end{matrix}$

The deflector velocity 410 (W) in relation to water coordinate systemmay be defined as follows:

$\begin{matrix}{W = \begin{pmatrix}W_{x} \\W_{y}\end{pmatrix}} & (4)\end{matrix}$

Wherein the deflector velocity 410 (W) can be calculated according tothe following:

W=G−C  (5)

The velocity direction (D) of the deflector 150 in relationship to thewater coordinate system can be calculated according to the following:

$\begin{matrix}{D = {\arctan \left( \frac{W_{y}}{W_{x}} \right)}} & (6)\end{matrix}$

The angle of attack 300 (AoA) can then be calculated using the deflectorheading 405 (H) and the velocity direction D according to:

AoA=D−H  (7)

FIG. 5 illustrates one embodiment of a data processing system 400 thatmay be utilized in accordance with embodiments of the present invention.In some embodiments, the data processing system 400 may be a componentof the recording system 115 (e.g., FIG. 1). The data processing system400 may be used for implementing embodiments of the techniques forestimation of the angle of attack 300 (e.g., FIG. 3). Special or uniquesoftware for receiving the inputs, data processing, and sending outputsignals may be stored in the data processing system 400 and/or onexternal computer readable media. Those of ordinary skill in the artwill appreciate that the data processing system 400 may comprisehardware elements including circuitry, software elements includingcomputer code stored on a machine-readable medium or a combination ofboth hardware and software elements. Additionally, the blocks shown onFIG. 5 are but one example of blocks that may be implemented. Aprocessor 500, such as a central processing unit or CPU, may control theoverall operation of the data processing system 400. The processor 500may be connected to a memory controller 505, which may read data to andwrite data from a system memory 510. The memory controller 505 may havememory that includes a non-volatile memory region and a volatile memoryregion. The system memory 510 may be composed of a plurality of memorymodules, as will be appreciated by one of ordinary skill in the art. Inaddition, the system memory 510 may include non-volatile and volatileportions. A system basic input-output system (BIOS) may be stored in anon-volatile portion of the system memory 510. The system BIOS isadapted to control a start-up or boot process and to control thelow-level operation of the data processing system 400.

The processor 500 may be connected to at least one system bus 515 toallow communication between the processor 500 and other system devices.The system bus 515 may operate under a standard protocol such as avariation of the Peripheral Component Interconnect (PCI) bus or thelike. In the example embodiment shown in FIG. 5, the system bus 515 mayconnect the processor 500 to a hard disk drive 520, a graphicscontroller 525 and at least one input device 530. The hard disk drive520 may provide non-volatile storage to data that may be used by thedata processing system 400. The graphics controller 525 may be in turnconnected to a display device 535, which may provide an image to a userbased on activities performed by the data processing system 400. Thememory devices of the data processing system 400, including the systemmemory 510 and the hard disk drive 520 may be tangible, machine-readablemedia that store computer-readable instructions to cause the processor500 to perform a method according to an embodiment of the presenttechniques.

If there is a conflict in the usages of a word or term in thisspecification and or more patent or other documents that may beincorporated herein by reference, the definitions that are consistentwith this specification should be adopted for the purposes ofunderstanding this invention.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Various advantages of the present disclosurehave been described herein, but embodiments may provide some, all, ornone of such advantages, or may provide other advantages.

What is claimed:
 1. A method comprising: towing a plurality of streamers behind a survey vessel in a body of water, wherein at least one deflector provides a lateral component of force to the streamers; determining a deflector heading over ground using at least measurements from a heading sensor on a surface reference corresponding to the deflector; determining a deflector velocity over ground using at least measurements from a position sensor on the surface reference; determining a water current of the body of water; and estimating a deflector angle of attack based on inputs comprising the deflector heading, the deflector velocity over ground, and the water current.
 2. The method of claim 1, further comprising determining a deflector lift and comparing the deflector angle of attack and the deflector lift.
 3. The method of claim 2, further comprising sending a signal to the deflector to adjust the actual deflector angle of attack in response to the step of comparing the deflector angle of attack and the deflector lift.
 4. The method of claim 2, wherein the deflector lift is determined using at least measurements from a tension sensor on a lead-in line for the deflector.
 5. The method of claim 2, further comprising cleaning the deflector in response to the step of comparing the deflector angle of attack and the deflector lift.
 6. The method of claim 1, wherein the water current is determined using at least measurements from a water current measurement system on the survey vessel.
 7. The method of claim 1, further comprising adjusting the deflector angle of attack to modify lateral thrust generated by the deflector in response to the step of estimating the deflector angle of attack.
 8. The method of claim 1, wherein heading sensors comprises a global positioning system sensor, and wherein the surface reference comprises a float that supports the deflector in the body of water.
 9. The method of claim 1, wherein the position sensor comprise a magnetometer.
 10. The method of claim 1, wherein the deflector comprises at least one of a door, a paravane, or a steering rudder
 11. The method of claim 1, wherein the heading sensor comprises at least one of a compass, a gyroscope, or a magnetometer.
 12. A system comprising: a heading sensor on a surface reference corresponding to a deflector; a position sensor on the surface reference; a current measurement system on a survey vessel; and a data processing system communicatively coupled to the heading sensor, position sensor, and current measurement system, wherein the data processing system is configured to estimate a deflector angle of attack for the deflector based on inputs comprising measurements from the heading sensor indicative of deflector heading, measurements from the position sensor indicative of deflector velocity over ground, and measurements from the current measurement system indicative of water current.
 13. The system of claim 12, further comprising a tension sensor on a lead-in line for the deflector.
 14. The system of claim 12, wherein the data processing system is further configured to adjust the deflector angle of attack to modify lateral thrust generated by the deflector in response to the estimated deflector angle of attack.
 15. The system of claim 12, wherein the heading sensor comprises a global positioning system sensor, and wherein the surface reference comprises a float configured to support the deflector in a body of water.
 16. The system of claim 12, wherein the position sensor comprise a magnetometer.
 17. The system of claim 12, wherein the deflector comprises at least one of a door, a paravane, or a steering rudder.
 18. The system of claim 12, wherein the heading sensor comprises at least one of a compass, a gyroscope, or a magnetometer.
 19. A system comprising: a survey vessel; a current measurement system on the survey vessel; a streamer coupled to the survey vessel for towing behind the survey vessel, wherein geophysical sensors are disposed on the streamer at spaced apart locations; a deflector coupled to the streamer; a surface reference coupled to the deflector for supporting the deflector in a body of water; a heading sensor on the surface reference; and a position sensor on the surface reference.
 20. The system of claim 19, further comprising a data processing system communicatively coupled to the heading sensor, position sensor, and current measurement system, wherein the data processing system is configured to estimate a deflector angle of attack for the deflector based on inputs comprising measurements from the heading sensor indicative of deflector heading, measurements from the position sensor indicative of deflector velocity over ground, and measurements from the current measurement system indicative of water current.
 21. The system of claim 19, further comprising a tension sensor on a lead-in line for the deflector.
 22. The system of claim 19, wherein the data processing system is further configured to adjust the deflector angle of attack to modify lateral thrust generated by the deflector in response to the estimated deflector angle of attack.
 23. The system of claim 19, wherein the heading sensor comprises a global positioning system sensor, and wherein the surface reference comprises a float configured to support the deflector in a body of water.
 24. The system of claim 19, wherein the position sensor comprise a magnetometer.
 25. The system of claim 19, wherein the deflector comprises at least one of a door, a paravane, or a steering rudder.
 26. The system of claim 19, wherein the heading sensor comprises at least one of a compass, a gyroscope, or a magnetometer. 